The present invention relates to methods for forming bevel gear shafts. In particular, the present invention relates to a method for forming a bevel gear shaft having a desired recess shape, such as an involute spline, in its longitudinal direction.
Known methods for producing bevel gear shafts are useful for a variety of applications. The methods can be applied to the production of drive pinions on limited slip differential gears. For example, referring to FIG. 7 of the present invention, a bevel gear I having an involute spline 2 is utilized as a driving pinion on a limited slip differential gear.
Conventionally, when manufacturing bevel gear shafts, involute spline 2 is produced by a material removal process such as machining. The main disadvantages of material removal processes are low productivity and decreased mechanical strength. Forming involute spline 2 by machining requires 1-5 minutes and the resulting structure has only 75-80% of the mechanical strength of the same structure formed by forging. This lower mechanical strength of machined -vs- forged structures is known to be due to the interruption of the stabilized grain structure by the shearing action of the machining process.
Referring now to FIG. 8, when the involute spline is produced on a bevel gear shaft by conventional forging techniques, a shoulder 4 and an approach angle 5 are required for guiding the blank into the die, for stabilizing it during the punching process, and for drawing the shaft in the bevel gear. Therefore, forging an involute spline on the entire length of the shaft with no shoulder 4 and no approach angle 5, as in FIG. 7., cannot be accomplished with a conventional die forging process.
Referring now to FIG. 9, involute splines 2 can be formed before producing bevel gear portion 3. However, in order to make the involute spline fit smoothly in a gear die, the involute spline is required to be of smaller dimension than gear die. Moreover, in the next step the involute spline is required to be uniformly enlarged. It is impossible to accurately enlarge the spline uniformly. Furthermore, with this process a protruding portion 6 is produced in forming the bevel gear portion, thereby reducing the yield rate.